Fundus camera imaging is acknowledged to be an important diagnostic tool for detection of various conditions affecting the eye, including diabetic retinopathy and macular degeneration. Various embodiments of fundus imaging apparatus are disclosed, for example in U.S. Pat. No. 5,713,047 (Kohayakawa); U.S. Pat. No. 5,943,116 (Zeimer); U.S. Pat. No. 5,572,266 (Ohtsuka); U.S. Pat. No. 4,838,680 (Nunokawa); U.S. Pat. No. 6,546,198 (Ohtsuka); U.S. Pat. No. 6,636,696 (Saito); U.S. Pat. No. 4,247,176 (Ito); U.S. Pat. No. 5,742,374 (Nanjo et al.); and U.S. Pat. No. 6,296,358 (Comsweet et al.)
While these patents attest to continuous improvements in fundus camera design, there are still significant hurdles to obtaining good quality images from these devices. Fundus cameras must solve the fairly difficult problem of simultaneously illuminating the retina through the pupil and obtaining the retinal image, with both illumination and image-bearing light traveling along substantially the same optical path. One particularly troublesome problem relates to the need to provide illumination at levels sufficient for imaging and, at the same time, to eliminate stray light caused by unwanted reflection from the cornea that surrounds the pupil. It is difficult to obtain both a wide field of view and sufficient illumination while keeping unwanted reflected light from the optical system.
This problem is most readily illustrated by an overview of the operation of the illumination subsystem in a conventional fundus imaging apparatus. Referring to FIG. 1, there is shown a fundus imaging apparatus 10 in which a conventional illumination section 12 is used. The patient's eye E is positioned along an optical axis O using an alignment subsystem (not shown in FIG. 1). Illumination section 12 directs light either from an observation light source 14 and a lens 16 or from an image capture light source 18 and a lens 20 as controlled by control logic circuitry for fundus imaging apparatus 10 (not shown in FIG. 1). A half-mirror 22 directs light from the appropriate source through a ring-slit diaphragm 24 and a lens 26, to an apertured mirror 28. Apertured mirror 28 directs the illumination light along axis O and through an objective lens 42 toward the pupil for illuminating the retina of eye E. Depending on the use of fundus imaging apparatus 10 at any one time, either observation light source 14 or image capture light source 18 are activated. Observation light source 14 is typically infrared (IR) light, to which eye E is insensitive. Image capture light source 18, on the other hand, may be a high-brightness source such as a xenon lamp, for example. Depending on the application, image capture light source 18 may be pulsed or strobed.
Ring-slit diaphragm 24 has the characteristic functional arrangement shown in FIG. 2. Light is transmitted through an inner ring 30 and is blocked at a middle section 32 and at an outer section 34. As is shown in the received illumination ring of FIG. 3, inner ring 30, having an inner radius 41, is directed into a pupil 36 of the patient as a ring 40 of illumination. To obtain the retinal image, apertured mirror 28 (FIG. 1) has an aperture suitably centered about optical axis O to allow light that has been reflected from the retina of eye E and directed through lenses 42 and 44 to reach a sensor 46, such as a charge coupled device (CCD).
The high-level block diagram of FIG. 1 thus gives an overview of illumination section 12 that applies for conventional fundus imaging apparatus. There have been numerous methods disclosed for optimizing the performance of illumination section 12, including components arranged to prevent stray reflected light from the cornea of eye E and from optical surfaces from being directed back toward sensor 46. Referring to the schematic block diagram of FIG. 1, three basic approaches have been followed in order to reduce or eliminate stray light from these sources:                (i) Using a pair of crossed polarizers. Using this approach, a first polarizer 110 is placed in the illumination path, at some position before apertured mirror 28. A second polarizer 112 is then positioned in the image path, at some point between apertured mirror 28 and sensor 46. With reference to FIG. 1, possible locations of first polarizer 110 and second polarizer 112 are shown in phantom. The two polarizers 110 and 112 would be oriented with transmission axes orthogonally disposed with respect to each other.        There are two key problems with this method. The first problem relates to the needed lamp power when using this strategy. Because only that portion of light having the proper polarization is transmitted through polarizer 110, more light is needed from image capture light source 18. The use of second polarizer 112 further reduces the available power. As a result, image capture light source 18 must be about 4 times as bright as would be necessary without polarizers 110 and 112. The second problem relates to the nature of light reflected from the cornea. Since a portion of this light can be depolarized, particularly due to the large incident angle, second polarizer 112 will be less effective in blocking unwanted stray light.        (ii) Blocking light that would otherwise reflect back from the surface of objective lens 42. This solution, however, reduces uniformity of the desired light reflected from the retina, particularly noticeable when attempting to obtain retinal images from near-sighted patients.        (iii) Separating illumination and imaging optical paths. A beamsplitter can be placed in front of objective lens 42 to effect this separation. However, this type of solution requires additional lamp power in order to obtain suitable reflected light from the retina and necessitates a longer working distance for objective lens 42.        
In order to provide uniform illumination without unwanted reflected light from the cornea at the same time, as described with reference to FIGS. 2 and 3, conventional fundus imaging systems require pupil dilation. The disadvantages of pupil dilation include patient inconvenience, lost time, and discomfort. Attempts to design these devices for use with un-dilated pupils have been largely unsuccessful to date, resulting in limited field of view and insufficient illumination for accurate imaging. If the pupil is not dilated, the field of view of the camera can be limited to no more than about 30 degrees, for example, when the pupil diameter is about 3 mm.
As yet another limitation, the size of the illumination ring in conventional fundus imaging systems is either fixed at a single value or is switchable among a small set of discrete values, obtained by inserting different masks or apertures into the optical path. There is no capability for changing the size of the illumination ring in a continuous fashion.
Thus, it can be seen that there is a need for a fundus imaging apparatus having illumination and imaging optics that allow imaging with smaller pupil diameters in order to reduce or eliminate the requirement for pupil dilation. Such a system should provide a suitably wide field of view, adjustability of illumination ring diameter, and good image quality.